At a start of a fuel cell stack in a fuel cell system, a conventionally proposed technique stops the operation of a pump for cooling water and thereby stops circulation of the cooling water, when the internal temperature of the fuel cell is equal to or lower than 0° C. (for example, Patent Literature 1). This proposed technique increases the heat value immediately after a sub-zero start and increases the rate of temperature rise of the fuel cell stack, so as to shorten the starting time.